Inflatable safety restraint devices, or airbags, are mandatory on most new vehicles. Airbags are typically installed as part of a system with an airbag module in the steering wheel on the driver's side of car, and in the dashboard on the passenger side of a car. In the event of an accident, a sensor within the vehicle measures abnormal deceleration and triggers the ignition of a charge contained within an inflator. Expanding gases from the charge travel through conduits and fill the airbags, which immediately inflate in front of, or adjacent to, the driver and passenger to protect them from harmful impact with the interior of the car. Typically, airbags are concealed within the vehicle trim to be invisible during normal vehicle operation.
Recent safety standards issued by various governments require more comprehensive protection in the event of a side or frontal collision. Additionally, there is an ongoing need to provide airbag systems that are economical to manufacture and install, avoid interference with the ability of the automaker to position various features on the vehicle interior, and can be expected to deploy reliably. The passengers of a vehicle may be at any of a wide variety of positions within the vehicle at the time of deployment, so it is desirable to provide airbag systems that protect against impact over the broadest possible range of occupant locations.
Additional airbag systems, such as inflatable curtains and seat-mounted airbags have also been developed in response to the need for similar protection from lateral impacts between a passenger and the side of a vehicle's interior. This need might arise when another vehicle collides with the side of the car, or in a rollover situation in which the side of car is repeatedly impacting the ground. Inflatable curtain airbags are often stowed along the roof rail where the vehicle roof meets the side windows and pillars or within an outboard compartment of a vehicle seat. Other side impact airbag systems are stowed within a compartment located within the vehicle seat.
For any type of airbag including inflatable curtain and side impact airbags, one of the largest challenges is to provide the appropriate rigidity of the deployed cushion. The pressure of inflation gas within the deployed cushion may determine the rigidity of the cushion. A cushion that is not rigid enough may risk injury as the occupant's body part fully compresses the cushion to impact the vehicle surface behind the cushion. A cushion that is too rigid could cause injury as the person's body part impacts the cushion itself.
A complicating factor is that different parts of the body have different levels of mass and rigidity. A more massive part of the body will require greater cushion stiffness to avoid impact of the body part with the vehicle surface. Similarly, a more rigid part of the body (such as a bony area) may also tend to require greater cushion rigidity as there is a greater likelihood that a rigid body part will press excessively against the adjoining portion of the cushion, and thereby impact the vehicle surface behind the cushion. This “strikethrough” may occur when the internal pressure of the airbag is insufficient to prevent the colliding occupant from passing through the total depth of the airbag and contacting the hard interior surface of the vehicle.